Buying an NAS. Help/advice

Hi. I am looking to upgrade the available space on my computer. I have been looking into NAS, but the only devices I see that would fit what I think my needs are are few, expensive and have vague reviews.

I have about 1TB of disk space right now (almost full). I want to have a backup of that, and a friend of mine suggested I look at NASs. Additionally, I estimate that I have 1TB +/- a few hundred gigs of data on dvds and cds that I want to put onto a hard drive (and back THAT up). So now that's 4TB all told. Then I figured I'd get 1TB for future space considerations. Backing that disc up would bring it all to 6TB.

I have a few questions: Do I need to get one NAS device that can take 6TBs? And if so are there diskless drives where I can put drives of my choice into that NAS.
Can I get, say, 3 2TB NAS devices and link them together? I was told to look at Netgear for the NAS and seagate for the drives. I have a Windows system and would not be installing this by myself. I have a computer tech who would do it for me.

Any other tips are appreciated. Thanks!